For a tamer example, it would be entirely possible to make a bot that responds to everything a particular person posts on Twitter with pictures of human poop. If you use public-domain images, like Wikimedia's, it's even legal. But no one wants to use a website that's plagued by Poop-Bot and its ilk. A site that allowed Poop-Bot to run rampant would lose all its users in short order.

I somehow think, even though it's just gross and not actually dangerous, Poop-Bot would get kicked off the vast majority of "free speech" sites.